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| United States Patent |
6,165,412
|
|
Lunk
, et al.
|
December 26, 2000
|
Method of making non-sag tungsten wire for electric lamps
Abstract
A method of making a potassium-doped tungsten powder is described
comprising forming a mixture of ammonium paratungstate or ammonium
metatungstate and a potassium-containing compound selected from a
thermally unstable potassium-containing salt or a potassium tungstate, and
reducing the mixture in a single step without adding additional dopants to
form a potassium-doped tungsten powder. The potassium-doped tungsten
powder produced by the method of this invention can be pressed, sintered
and drawn to produce a non-sag tungsten wire.
| Inventors:
|
Lunk; Hans-Joachim (Towanda, PA);
Salmen; Michael (Brandenburg, DE);
Stevens; Henry J. (Athens, PA)
|
| Assignee:
|
Osram Sylvania Inc. (Danvers, OH)
|
| Appl. No.:
|
390201 |
| Filed:
|
September 7, 1999 |
| Current U.S. Class: |
419/4; 419/28; 419/34; 419/35; 419/38 |
| Intern'l Class: |
B22F 001/02 |
| Field of Search: |
419/4,34,38,35,28
|
References Cited
U.S. Patent Documents
| 1082933 | Dec., 1913 | Coolidge.
| |
| 1226470 | May., 1917 | Coolidge.
| |
| 1410499 | Mar., 1922 | Pacz.
| |
| 3853492 | Dec., 1974 | Millner et al. | 29/182.
|
| 3927989 | Dec., 1975 | Koo | 29/182.
|
| 4971757 | Nov., 1990 | Day et al. | 519/23.
|
| 5019330 | May., 1991 | Bewlay et al. | 419/39.
|
| 5072147 | Dec., 1991 | Pugh et al. | 313/341.
|
| 5087299 | Feb., 1992 | Fukuchi et al. | 148/11.
|
| 5284614 | Feb., 1994 | Chen et al. | 419/20.
|
| 5785731 | Jul., 1998 | Fait et al. | 75/368.
|
| 5795366 | Aug., 1998 | Salmen et al. | 75/368.
|
Other References
1 K. Hara et al., The Development of High Quality Tungsten Wire for High
Stress Halogen Lamp, Nippon Tungsten Review 29 (1997) pp. 20-29.
2 H.-J. Lunk et al., What is Behind "Tungsten Blue Oxides"?, Refractory
Metals & Hard Materials 12 (1993-1994) pp. 17-26.
3 H.-J. Lunk et al., Solid State 1H NMR Studies of Different Tungsten Blue
Oxides and Related Substances, Refractory Metals, & Hard Materials 16
(1198) pp. 3-30.
|
Primary Examiner: Jenkins; Daniel J.
Attorney, Agent or Firm: Clark; Robert F.
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATIONS
This application is related to commonly assigned application Ser. No.
09/391,121, filed Sep. 7, 1999 now allowed.
Claims
We claim:
1. A method of making a potassium-doped tungsten powder comprising:
forming a dry mixture of ammonium paratungstate or ammonium metatungstate
and a potassium-containing compound selected from a thermally unstable
potassium-containing salt or a potassium tungstate; and
reducing said mixture in a single step to form a potassium-doped tungsten
powder.
2. The method of claim 1 wherein the thermally unstable
potassium-containing salt is potassium nitrate, potassium nitrite,
potassium carbonate or potassium hydrogencarbonate.
3. The method of claim 1 wherein the potassium-containing compound is a
potassium tungstate having a general formula K.sub.2 W.sub.n O.sub.3n+1,
where n is from 1 to 8.
4. The method of claim 1 wherein the mixture is reduced in a
hydrogen-containing atmosphere at a temperature from about 500.degree. C.
to about 1000.degree. C.
5. The method of claim 1 wherein the potassium-containing compound is
potassium nitrate or K.sub.2 WO.sub.4.
6. The method of claim 5 wherein the mixture is reduced in a
hydrogen-containing atmosphere at a temperature from about 500.degree. C.
to about 1000.degree. C.
7. A method of making a sintered compact of non-sag tungsten comprising:
forming a dry mixture of ammonium paratungstate or ammonium metatungstate
and a potassium-containing compound selected from a thermally unstable
potassium-containing salt or a potassium tungstate;
reducing said mixture in a single step to form a potassium-doped tungsten
powder;
forming the potassium-doped tungsten powder into a pressed compact; and
sintering the pressed compact at a temperature from about 1600.degree. C.
to about 2000.degree. C.
8. The method of claim 7 wherein the thermally unstable potassium
containing salt is potassium nitrate, potassium nitrite, potassium
carbonate or potassium hydrogencarbonate.
9. The method of claim 7 wherein the potassium-containing compound is a
potassium tungstate having a general formula K.sub.2 W.sub.n O.sub.3n+1,
where n is from 1 to 8.
10. The method of claim 7 wherein the mixture is reduced in a
hydrogen-containing atmosphere at a temperature from about 500.degree. C.
to about 1000.degree. C.
11. The method of claim 7 wherein the potassium-containing compound is
potassium nitrate or K.sub.2 WO.sub.4.
12. The method of claim 11 wherein the mixture is reduced in a
hydrogen-containing atmosphere at a temperature from about 500.degree. C.
to about 1000.degree. C.
Description
TECHNICAL FIELD
This invention relates to non-sag tungsten wire for use as filaments in
electric lamps. In another aspect, this invention relates to methods of
making potassium-doped tungsten powder for non-sag tungsten wire.
BACKGROUND ART
The metallurgy of tungsten plays a central role in the development of lamp
filaments. Tungsten wire is made in various stages in accordance with the
well-known Coolidge method introduced in 1910; U.S. Pat. No. 1,082,933
(1913) and U.S. Pat. No. 1,226,470 (1917).
Pure tungsten wire is not suitable to make filaments for incandescent
lamps. Under typical operating conditions, the individual grains of the
filament have the tendency to offset, or slide off (creep or sag) with
respect to each other. This causes the filament to sag and short out. A
lamp made with such filaments will, therefore, fail prematurely. The
beneficial effects of doping to improve the creep resistance of tungsten
wire were recognized as early as 1910, and doping was practiced
henceforth. Systematic doping of tungsten oxide powder with
potassium-containing chemicals was patented by Pacz in 1922, U.S. Pat. No.
1,410,499. Non-sag (NS) tungsten wire is unique in that it is a composite
between two mutually insoluble metals, tungsten and potassium. The non-sag
properties are attributed to longitudinal rows of sub-microscopic bubbles
containing liquid and/or gaseous potassium.
The long chain of processes in a standard powder metallurgical (P/M)
manufacturing of potassium-doped tungsten wire starts with the partial
reduction of ammonium paratungstate tetrahydrate (APT), (NH.sub.4).sub.10
[H.sub.2 W.sub.12 O.sub.42 ].4H.sub.2 O, in hydrogen or hydrogen/nitrogen,
to produce `tungsten blue oxide` (TBO). The composition of the
blue-colored TBO, having the general formula xNH.sub.3.yH.sub.2 O.WO.sub.n
(0<x<0.1, 0<y<0.2, and 2.5<n<3.0), depends on the reduction conditions of
APT such as temperature, atmosphere, type of rotary kiln or pusher-type
furnace and feed rate through the furnace. Along with crystalline
compounds (WO.sub.3, W.sub.20 O.sub.58, W.sub.18 O.sub.49, WO.sub.2 and
hexagonal tungsten bronze phases, A.sub.x B.sub.y WO.sub.3 (A=NH.sub.4,
H.sub.3 O; B=NH.sub.3, H.sub.2 O; 0<x+y<0.33)), industrially produced TBO
powders may contain up to 50% of amorphous phases. The TBO is doped with
aqueous solutions of potassium silicate (1500-2500 ppm K, 1500-2500 ppm
Si) and aluminum nitrate (or aluminum chloride) (.about.300 ppm Al). It is
then dried and milled. The doped TBO is then reduced in hydrogen to metal
powder. By some manufacturers, a separate "browning" step (reduction to
.about.`WO.sub.1 `) is used. The doped tungsten powder is washed first
with water, then with hydrofluoric and hydrochloric acid to remove
unnecessary and undesired amounts of dopants. The powder is then dried in
air. Appropriate powder blends are made to give a potassium content of >90
ppm in an acid-washed sample of powder. The washed powder is then
mechanically or isostatically pressed and sintered by high-temperature
resistance sintering at above 2900.degree. C. The ingots which have a
density of >17.0 g/cm.sup.3 and a K content of >60 ppm are rolled or
swaged, and finally drawn into wire.
The multi-step process leads to the outstanding high-temperature creep
resistance of NS tungsten wire. It is generally recognized that the NS
tungsten wire should have a potassium content of at least 60 ppm.
Furthermore, it has been proposed that a potassium content of 80 ppm or
higher, and in particular 85-110 ppm K, is necessary for high performance
NS tungsten wire. See, e.g., K. Hara, et al., The Development of High
Quality Tungsten Wire for High Stress Halogen Lamp, Nippon Tungsten Review
29 (1997), pp. 20-29.
The silicon and aluminum added to the TBO in the standard method serve
exclusively as `helpers` during the reduction and sintering stages. After
high-temperature sintering their concentration is reduced to less than 10
ppm each. Neither element plays any positive role in the final NS tungsten
wire. In addition, the number of steps in the standard process makes the
process inefficient and the process further produces a contaminated acid
waste, which must be properly disposed of. Hence, it would advantageous to
have a more efficient method of making NS tungsten wire, which used fewer
processing steps and did not produce an acid waste.
SUMMARY OF THE INVENTION
It is an object of the invention to obviate the disadvantages of the prior
art.
It is another object of the invention to provide a simpler method of
producing non-sag tungsten for use in lighting applications.
It is a further object of the invention to provide a lower cost, less
environmentally sensitive method for producing non-sag tungsten.
In accordance with one aspect of the invention, there is provided a method
of making a potassium-doped tungsten powder.
The method comprises forming a mixture of ammonium paratungstate or
ammonium metatungstate and a potassium-containing compound selected from a
thermally unstable potassium-containing salt or a potassium tungstate, and
reducing said mixture in a single step (single step reduction) to form a
potassium-doped tungsten powder.
In accordance with another aspect of the invention, the method further
includes forming the potassium-doped tungsten powder into a pressed
compact and sintering the pressed compact at a temperature from about
1600.degree. C. to about 2000.degree. C.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
For a better understanding of the present invention, together with other
and further objects, advantages and capabilities thereof, reference is
made to the following disclosure and appended claims.
The present invention involves the dry doping of ammonium paratungstate
tetrahydrate (APT), (NH.sub.4).sub.10 [H.sub.2 W.sub.12 O.sub.42
].4H.sub.2 O and ammonium metatungstate hydrate (AMT), (NH.sub.4).sub.6
[H.sub.2 W.sub.12 O.sub.40 ].nH.sub.2 O with potassium-containing salts
and a subsequent single step reduction of the mixture in hydrogen to yield
potassium-doped tungsten powders. Preferably, the reduction is performed
at temperatures from about 500.degree. C. to about 1000.degree. C. As used
herein, the term single step reduction means that the reduction is not
interrupted to include an additional doping step. The single step
reduction does not exclude using multiple firing conditions
(time/temperature/atmosphere) during the reduction step.
The potassium-doped tungsten powders are usable directly in the standard
P/M manufacturing of tungsten wire for incandescent lamps without acid
washing. The preferred potassium salts used in this invention are
potassium nitrate, KNO.sub.3, and potassium nitrite, KNO.sub.2, although
it is anticipated that any thermally unstable potassium-containing salt or
any one of a number of different potassium tungstates may be used as a
doping compound. For example, potassium carbonate, K.sub.2 CO.sub.3, or
potassium hydrogencarbonate, KHCO.sub.3, and also potassium tungstates,
K.sub.2 W.sub.n O.sub.3n+1 (n=1 to 8), can be useful as doping compounds.
Another novel feature of the invention is that the potassium-doped tungsten
may be sintered into dense ingots at distinctly lower temperatures than
the prior art processes. In particular, the potassium-doped tungsten
powder may be sintered at temperatures from about 1600.degree. C. to about
2000.degree. C. The process of this invention enables the production of
sintered tungsten ingots containing potassium in amounts up to 120 ppm and
densities higher than 17.0 g/cm.sup.3.
The novel process eliminates the prior art steps of wet doping TBO with
aluminum- and silicon-containing chemicals, drying and milling of the
doped TBO, acid washing of the tungsten powder, and high sintering
temperatures. As a result the process is more economical and less harmful
to the environment.
The following non-limiting examples are presented.
EXAMPLE 1
Three thousand grams of reagent grade APT (`Medium APT` with sifting
characteristics of 15 to 25% -325 mesh and 30 to 40% -200 mesh) was
carefully blended with 4.8 g of ground potassium nitrate, KNO.sub.3.
First, the total amount of KNO.sub.3 was added to 100 g of the APT in a
125 ml plastic bottle and homogenized on a roller blender for two hours.
Then the mixture was transferred into a 500 ml plastic bottle, mixed with
500 g of APT and rolled for another two hours. The final blending step was
provided in a 2 l plastic bottle by mixing the intermediate blend with the
remaining 2400 g of APT and rolling the powders for two hours.
Three hundred gram samples of potassium-doped APT were reduced in a
laboratory LINDBERG furnace in an 11" Inconel boat under the following
conditions: 30 cfh dry hydrogen, a heating rate of 6 K/min, a one hour
hold at 550.degree. C. and a final one hour reduction time at 900.degree.
C. The tungsten powder was then mechanically pressed into 9 g compacts and
sintered in hydrogen at 1800.degree. C. for six hours.
EXAMPLE 2
Sintered compacts of NS tungsten were prepared as in Example 1 except that
7.8 g of dried and ground potassium tungstate, K.sub.2 WO.sub.4, was used
in place of potassium nitrate.
EXAMPLE 3
Sintered compacts of NS tungsten were prepared as in Example 1 except that
AMT was used in place of APT.
The characteristics of the potassium-doped tungsten powders and sintered
compacts are given in Table 1. As can be seen, water washing of the doped
tungsten powder significantly reduced the amount of potassium. However,
the potassium retention of the sintered compact did not depend on whether
the doped tungsten powder was or was not water washed prior to sintering.
Nor was there any significant difference between the measured densities of
the sintered compacts made with unwashed tungsten powder (unwashed) and
the ones made with water washed tungsten powder (washed).
TABLE 1
__________________________________________________________________________
FSSS (.mu.m)
Potassium (ppm) Hg density (g/cm.sup.3)
Water Sintered
Water
Sintered
Sintered
Sintered
Unwashed Washed Unwashed Compact Washed Compact Compact Compact
Sample Powder Powder Powder
(Unwashed) Powder (Washed)
(Unwashed) (Washed)
__________________________________________________________________________
3000 g APT +
1.75 1.70
242 120 179 121 17.4 17.5
4.8 g KNO.sub.3
3000 g APT + 1.67 1.63 238 80 108 82 17.5 17.6
7.8 g K.sub.2 WO.sub.4
3000 g AMT + 1.48 1.42 165 49 85 49 18.0 18.2
4.8 g KNO.sub.3
__________________________________________________________________________
While there has been shown and described what are at the present considered
the preferred embodiments of the invention, it will be obvious to those
skilled in the art that various changes and modifications may be made
therein without departing from the scope of the invention as defined by
the appended claims.
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